Optoelectronic devices generally include light-emitting devices and photovoltaic devices. These devices generally include an active layer sandwiched between two electrodes, sometimes referred to as the front and back electrodes, at least one of which is typically transparent. The active layer typically includes one or more semiconductor materials. In a light-emitting device, e.g., an organic light-emitting diode (OLED) device, a voltage applied between the two electrodes causes a current to flow through the active layer. The current causes the active layer to emit light. In a photovoltaic device, e.g., a solar cell, the active layer absorbs energy from light and converts it to electrical energy which generates a flow of current at some characteristic voltage between the two electrodes.
The light is transmitted through at least one of the electrodes of an OLED device. The design of a suitable transparent electrode requires that it provide in-plane electrical conductivity (favoring a thicker layer of material) and that it provide optical transmission through its thickness (favoring a thinner layer of material). To resolve these opposing constraints on the electrode design, it is preferred to limit the size of individual light emitting regions (pixels), and thus limit the amount of current that is flowing laterally in the plane of the electrode. If the current is low, the resistive losses in the electrode are low and the resulting device is efficient. In the one case, a pixel is defined by unlit lines that define its perimeter, and the current is bused to these regions. In another case, the pixel may be defined by points that define its perimeter, and current is bused to the electrode(s) at these points. In either case, the unlit regions interrupt the otherwise uniform appearance of an OLED. The typical maximum dimension for a pixel in the direction of current flow is on the order of 1 cm before excessive loss and non-uniform appearance results. Approaches to solving this problem include making the unlit regions very small (increasing the complexity of the manufacturing process) or to obscure them with a diffusing film (reducing efficiency and adding cost). Thus it is desirable to decrease the appearance of unlit regions so that large uninterrupted areas light can be created. More generally, it is desirable to configure large arrays of lighted areas from individual pixels while providing design flexibility. Accordingly, it is desirable to configure pixels in an arbitrary ordered on disordered array patterns, to introduce pixels of different size, shape, color and brightness, and spacing, and also to be able to replace individual pixels in the array.